(1). Field of the Invention
The present invention relates to a processor with VLIW (Very Long Instruction Word) architecture, and in particular to a processor that executes instructions with comparatively short word length and high code efficiency.
(2). Description of the Prior Art
With the increase in demand for multimedia devices and the miniaturization of electronic circuits in recent years, there has been a growing need for microprocessors that can process multimedia data, such as audio data and image data, at high speed. One kind of processors that are capable of meeting this need are processors that use VLIW architecture, these being hereinafter referred to as xe2x80x9cVLIW processorsxe2x80x9d.
VLIW processors include a number of internal operation units and so are able to simultaneously execute a number of operations in one VLIW in parallel. Such VLIW are generated by a compiler that investigates the extent to which parallel processing is possible at the source program level and performs scheduling. For embedded microprocessors used in consumer appliances, however, it is important to suppress the code size of programs, so that 256-bit VLIW, with their high incidence of no-operation instructions (hereinafter referred to as xe2x80x9cNOP instructionsxe2x80x9d) and resulting poor code efficiency, are far from ideal.
One example of a VLIW processor that executes instructions with relative short word length is Japanese Laid-Open Patent Application H09-26878. This technique teaches a data processing apparatus that is a VLIW processor for executing 32-bit instructions that can simultaneously indicate a maximum of two operations.
FIGS. 1A and 1B show the instruction format of the stated technique, with FIG. 1A showing the instruction format for simultaneously indicating two operations and FIG. 1B showing the instruction format for indicating only one operation. This technique aims to improve code efficiency by including a 2-bit value in the format field 410 that shows the number of operations in each instruction and the execution order.
The indication of a maximum of two operations by a single 32-bit instruction, however, does not achieve a sufficient degree of parallelism. There is also the problem of decreases in code efficiency of instructions when performing an operation using a constant that exceeds a given word length. As one example, when a 32-bit constant is split into an upper 16 bits and a lower 16 bits so that it can be set into registers, two 32-bit instructions are required just to indicate an operation using this constant.
In view of the stated problems, it is a first object of the present invention to provide a VLIW processor that executes instructions of comparatively short word length, but which have a high degree of parallelism and a highly efficient code structure so that several operations can be simultaneously indicated. As one example, three or more operations can be indicated by a single 32-bit instruction.
It is a second object of the present invention to provide a VLIW processor for executing instructions of a comparatively short word length that have a structure whereby the overall code efficiency will be comparatively unaffected even when handling constants of comparatively long word length.
The first object can be realized by a VLIW (Very Long Instruction Word) processor that decodes and executes an instruction that has at least two operation fields, of which a first operation field can only include one operation code for specifying an operation type and a second operation field includes a combination of one operation code and at least one operand used in an operation indicated by the second operation field, the VLIW processor including: a first decoding unit for decoding the operation code in the first operation field; a first execution unit for executing an operation indicated by the operation code in the first operation field in accordance with a decoding result of the first decoding unit; a second decoding unit for decoding the operation code in the second operation field; and a second execution unit for executing the operation indicated by the operation code in the second operation field on data which is indicated by the operands in the second operation field, in accordance with a decoding result of the second decoding unit.
By doing so, since at least one operation in the instruction can be indicated by merely inserting an operation code without an explicit indication of an operand, the word length of instructions can be reduced. As a result, a VLIW processor that executes instructions of comparatively short word length, but which have a highly efficient code structure so that several operations can be simultaneously indicated is achieved.
Here, a number of bits occupied by the operation code in the first operation field may be equal to a number of bits occupied by the operation code in the second operation field.
As a result, all operation codes that are included in an instruction will be composed of the same number of bits, which simplifies components such as the decoder circuits.
Here, the instruction may include three operation fields, wherein a third operation field in the three operation fields may occupy a same number of bits as the second operation field and may include a combination of one operation code and at least one operand, the VLIW processor further including: a third decoding unit which decodes, when an operation code is present in the third operation field, the operation code in the third operation field; and a third executing unit for executing an operation indicated by the operation code in the third operation field on data which is indicated by the operands in the third operation field, in accordance with a decoding result of the third decoding unit.
As a result, a VLIW processor with a high degree of parallelism whereby three operations can be simultaneously performed can be achieved.
Here, the first executing unit may control a control flow of a program including the instruction.
As a result, branch operations which do not normally require a large number of bits can be assigned to a short operation field. This means an instruction set with high code efficiency can be defined.
Here, the second executing unit may control transfer of the data that is indicated by the operands included in the second operation field, and the third executing unit may control executes an arithmetic logic operation on the data that is indicated by the operands included in the third operation field.
As a result, data transfer to and from an external memory can be indicated by a single operation in an instruction, so that the operand access circuit that should be provided in a VLIW processor can be simplified.
The second object of the present invention can be achieved by a VLIW processor that decodes and executes an instruction that has at least two operation fields, of which a first operation field can only include one of (i) a single operation code for specifying an operation type and (ii) a constant, and a second operation field includes one of (i) a combination of one operation code and at least one operand used in an operation indicated by the second operation field and (ii) a constant, the VLIW processor including: a first decoding unit which decodes, when an operation code is present in the first operation field, the operation code in the first operation field; a first executing unit for executing an operation indicated by the operation code in the first operation field, in accordance with a decoding result of the first decoding unit; a second decoding unit which decodes, when an operation code is present in the second operation field, the operation code in the second operation field; and a second executing unit for executing an operation indicated by the operation code in the second operation field on data which is indicated by the operands in the second operation field, in accordance with a decoding result of the second decoding unit.
With the stated construction, when it is necessary to put meaningless code into an operation field in an instruction, a constant that will be used by a different operation may instead be inserted, so that a VLIW processor can be realized for executing instructions which have a high code efficiency despite having only a short word length.
Here, the instruction also includes a format field including a format code indicating whether only a constant is located in the first operation field and whether only a constant is located in the second operation field, the VLIW processor further including: a format decoding unit for decoding the format code; and a constant storage unit for extracting, when a decoding result of the format decoding unit shows that only a constant is present in at least one of the first operation field and the second operation field, the constant in the instruction and storing the extracted constant.
As a result, constants placed in an operation field can be stored in the constant storage unit for use by an operation in a later instruction, so that decreases in code efficiency can be avoided even when handling constants of a comparatively long word length using instructions of a comparatively short word length.
Here, the format field, the first operation field, the operation code in the second operation field, each operand in the second operation field, the operation code in the third operation field, and each operand in the third operation field may each occupy n bits in the instruction.
With the stated construction, all of the fields that compose an instruction have the same number of bits, which enables the internal circuits of the VLIW processor to be simplified.
Here, a VLIW processor may include: a fetch unit for fetching an L-bit instruction that includes n operation fields; and n operation units which are each associated with a different one of the n operation fields in the fetched instruction and each independently execute an operation indicated in the associated operation field in parallel with each other; the VLIW processor being characterized by the n operation fields not all being a same size, and by L not being an integer multiple of n.
With the stated construction, there is no need for all of the operation fields in an instruction to have the same word length, making it possible to define instructions with high code efficiency. As a result, a VLIW processor that executes instructions of comparatively short word length, but which have a highly efficient code structure so that several operations can be simultaneously indicated is achieved.
Here, n may be 3 and L may be 32.
The stated construction realizes a VLIW processor with a high degree of parallelism whereby three operations that are specified by a single 32-bit instruction can be simultaneously performed.
Here, a number of operands included in at least one operation field out of the n operation fields may be different to a number of operands in other operation fields in the n operation fields.
With the stated construction, there is no need for every operation field in an instruction to have the same number of operands, so that instruction formats with a high degree of code efficiency can be defined.
Here, the n operation fields may include at least one operation field composed of only an operation code and at least one operation field composed of an operation code and at least one operand.
With the stated construction, the instruction word length is shorter than the case when every operation field in an instruction contains a combination of an operation code and operands, so that a VLIW processor that executes instructions which have a highly efficient code construction can be realized.
As described above, the present invention realizes a VLIW processor that executes instructions of comparatively short word length but which have a highly efficient code structure that allows several operations to be specified by a single instruction. This effect is especially noticeable for embedded processors that process multimedia data.